(1) Field of the Invention
The present invention relates to a thermal head for recording letters, symbols and the like on a heat-sensitive recording medium such as a heat-sensitive recording paper. More particularly, the present invention relates to an improvement of a thermal head of the type where a glaze layer is partially formed below a heat-generating resistor, in which formation of short circuits among electrodes is prevented even in the presence of voids on a ceramic substrate and the resolving power or image sharpness of heat-sensitive recording is enhanced.
(2) Description of the Prior Art
A thermal head for heat-sensitive recording has heretofore been used in a thermal printer of a facsimile or computer, and as shown in FIG. 4, the conventional thermal head comprises a glaze layer 11 formed on a ceramic substrate 10, a plurality of head-generating resistors 12 arranged in a line at small intervals on the glaze layer 11, and common electrodes 13 and individual electrodes 14 connected to both the front and rear ends, relatively to the delivery direction P of the heat-sensitive recording medium, of the heat-generating resistors 12.
The glaze layer 11 exerts such functions that heat generated by application of electricity to the heat-generating resistors 12 is not allowed to escape into the ceramic substrate 10 and the temperature of the heat-generating resistors 12 is promptly elevated to a desired level, and when application of electricity to the heat-generating resistors 12 is stopped, the heat possessed by the heat-generating resistors 12 is discharged at a high efficiency and the heat-generating resistors 12 are promptly cooled. Accordingly, if pulse voltages are applied to the heat-generating resistors 12, desired letters or symbols are printed on the heat-sensitive recording paper.
The glaze layer is divided into a partial glaze layer partially formed on the ceramic substrate 10 as shown in FIG. 4 and an entire glaze layer formed on the entire surface of the ceramic substrate. In case of the entire glaze layer, since the surface of the thermal head is flat, when a heat-sensitive recording paper is pressed to the thermal head by a platen, the pressing force is dispersed and the contact pressure between the heat-sensitive recording paper and the heat-generating resistors is insufficient, and therefore, beautiful printing is impossible. Moreover, a large amount of glass is necessary for formation of the glaze layer and the cost is increased. In contrast, in case of the partial glaze layer, only the glaze layer-formed portion of the surface of the thermal head rises and heat-generating resistors are formed on this rising portion, and therefore, when a heat-sensitive recording paper is pressed to the thermal head by a platen, the above defect of the entire glaze layer is not caused. Namely, the heat-sensitive recording paper can be contacted with the heat-generating resistors by an appropriate pressing force and beautiful printing is possible. Moreover, the partial glaze layer is advantageous in that the amount of glass can be reduced and the manufacturing cost can be decreased. Because of these advantages, the partial glaze layer is mainly adopted for thermal heads at the present.
Recently, eight heat-generating resistors are formed in 1 mm (8 dot/mm), but there is a trend to form 16 heat-generating resistors in 1 mm (16 dot/mm) for forming sharp prints. In this case, the width (t) of the clearance between adjacent electrodes is very narrow and, for example, about 10 microns, and the following problem arises.
Namely, in a thermal head of the partial glaze layer, most of electrode for applying electricity to heat-generating resistors are directly formed on the ceramic substrate, not through the glaze layer. For formation of electrodes on the ceramic substrate, as shown in FIG. 5, a heat-generating resistor layer 15 and an electrode layer 16 are formed on the entire top surface of the ceramic substrate 10 according to the film-forming technique (see FIG. 5-B), and a photoresist film 17 is formed thereon by spin coating (see FIG. 5-C). Then, a photomask 18 having a predetermined pattern is placed on the photoresist film 17 and the assembly is exposed to light to effect reaction in the predetermined portion of the photoresist film 17 (see FIG. 5-D). Then, development is carried out, and the photoresist film reacted by the light exposure is removed (FIG. 5-E) and the exposed electrode layer 16 and heat-generating resistor layer 15 are etched (see FIG. 5-F). Finally, the remaining photoresist film 17 is removed.
In this case, however, many voids (empty pores) having a diameter of about 10 microns are present in the surface of the ceramic substrate (see FIG. 5-A), and if such voids are located in the area where the heat-generating resistor layer and electrode layer are removed by etching, as shown in FIG. 5-D, the thickness of the photoresist film 17 on the voids 19 is increased and the photoresist film 17 in this portion is not completely reacted. Accordingly, at the subsequent etching step, as shown in FIG. 5-F, the electrode layer 16 and heat-generating resistor layer 15 are not completely removed by etching, with the result that a short circuit is formed between adjacent heat-generating resistors or adjacent electrodes.